Cryostat configuration with cryocooler and gas gap heat transfer device

ABSTRACT

A cryostat configuration for keeping liquid helium comprises an outer jacket ( 1 ) surrounding a helium container ( 2 ) connected at at least two suspension tubes ( 3 ) to the outer jacket ( 1 ), and with a neck tube ( 4 ) whose upper warm end ( 5 ) is connected to the outer jacket ( 1 ) and whose lower cold end ( 6 ) is connected to the helium container ( 2 ) and into which a multi-stage cold head of a cryocooler ( 7 ) is installed, wherein the outer jacket ( 1 ), the helium container ( 2 ), the suspension tubes ( 3 ) and the neck tube ( 4 ) delimit an evacuated space, and the helium container ( 2 ) is surrounded by at least one radiation shield ( 8 ) which is connected in a heat-conducting fashion to the suspension tubes ( 3 ) and also to a contact surface ( 9 ) on the neck tube ( 4 ) of the helium container ( 2 ). The cryostat configuration is characterized by a gas gap ( 13 ) between one or more cold stages of the cold head ( 7 ) and one or more contact surfaces ( 9 ) in the neck tube ( 4 ) which are each connected in a heat-conducting manner to a radiation shield ( 8 ) via a fixed, rigid or flexible thermal bridge ( 12 ), heat being transferred through the gas gap ( 13 ) from the respective radiation shield ( 8 ) to the corresponding cold stage of the cold head ( 7 ). A cryostat configuration of this type ensures that no vibrations of the cold head ( 7 ) stages pass detectably into the cryostat configuration, wherein the quality of the thermal connection between the cold head ( 7 ) and the radiation shield(s) ( 8 ) is nevertheless sufficient.

This application claims Paris Convention priority of DE 10 2004 034 729.8 filed Jul. 17, 2004 the complete disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns a cryostat configuration for keeping liquid helium, comprising an outer jacket and a helium container installed therein, wherein the helium container is connected to the outer jacket via at least two suspension tubes, and with a neck tube whose upper warm end is connected to the outer jacket and whose lower cold end is connected to the helium container, as well as a multi-stage cold head of a cryocooler, wherein the outer jacket, the helium container, the suspension tubes and the neck tube delimit an evacuated space, and wherein the helium container is surrounded by at least one radiation shield which is connected in a thermally conducting fashion to the suspension tubes and to a contact surface on the neck tube of the helium container.

One possibility of integrating the cold head of a cryocooler in a cryostat configuration is to install the e.g. two-stage cold head in a separate vacuum chamber (as described e.g. in U.S. Pat. No. 5,613,367) or directly in the vacuum chamber of the cryostat (as described e.g. in U.S. Pat. No. 5,563,566) such that the first cold stage of the cold head is rigidly connected to a radiation shield and the second cold stage is connected in a heat-conducting manner to the helium container, either directly or via a fixed, rigid or flexible thermal bridge. The overall heat input into the helium container can be compensated for by re-condensation of the helium, which is evaporated due to external heat input, on the cold contact surface in the helium container to obtain loss-free operation of the system. Disadvantageously, the connection between the second cold stage and the helium container generally has a thermal resistance which cannot be neglected, and vibrations of this cold stage may furthermore be transferred to the helium container.

These disadvantages can be avoided by inserting the cold head into a neck tube which connects the external vacuum shell of the cryostat to the helium container and is correspondingly filled with helium gas, as is described e.g. in the document US2002/0002830A1. The first cold stage of the two-stage cold head is in fixed heat-conducting contact with a radiation shield and the second cold stage is freely suspended in the helium atmosphere to directly liquefy evaporated helium.

Since the cold head is surrounded by helium gas and since there is a temperature difference between the cold head and the neck tube wall or further structural elements of the neck tube, a considerable amount of; heat may be transferred into the cold head due to thermal conduction in the gas as well as convection currents. WO03036207 and WO03036190 therefore propose insulating the cold head tubes.

As described above, vibrations in the second cold stage of the cold head of the cryocooler are not transferred to the helium container if the cold head is installed directly in the neck tube connected to the helium container. However, a solid thermal bridge is conventionally used between the first cold stage of the cold head and the radiation shield. This thermal connection should be as “soft” as possible to minimize transfer of vibrations. Towards this end, thin foil packets or wire-braids made from copper or aluminium are generally used. These vibration-reducing measures are described in numerous documents (e.g. U.S. Pat. No. 5,129,232, U.S. Pat. No. 5,331,735, U.S. Pat. No. 5,317,879). The requirement for poor vibration transmission (thin, long wires) is thereby opposed by the requirement for good heat transfer (thick, short wires). Consequently, a compromise must be found, and it is not possible to completely prevent vibrations of the cryocooler cold head from passing into the cryostat. This is particularly disadvantageous if it is part of a highly sensitive apparatus such as a magnetic resonance spectrometer, in particular, a magnetic resonance imaging (MRI) apparatus or a nuclear magnetic resonance spectrometer (NMR).

It is the object of the present invention to propose a cryostat configuration with which the thermal connection between all cold stages of the cold head of a cryocooler and the cryostat configuration are designed such that no measurable vibrations of the cold stages pass into the cryostat configuration, wherein sufficient heat transfer between the cold head and cryostat configuration is nevertheless ensured.

SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention by a gas gap disposed between one or more cold stages of the cold head and one or more contact surfaces in the neck tube that are each connected in a heat-conducting manner to a radiation shield via a fixed, rigid or flexible thermal bridge, wherein heat from the respective radiation shield is guided via the gas gap into the corresponding cold stage of the cold head.

Heat transfer from a radiation shield to a cold stage of the cold head is therefore effected via a gas gap by guiding the heat transmitted to the radiation shield via the gas gap to the cold head. The inventive cryostat configuration has no fixed connection between the cold stage(s) of the cold head of the cryocooler and the radiation shield(s) such that transmission of vibrations from the cold head to the radiation shield(s) is largely eliminated, while nevertheless ensuring good thermal contact between the cold head and the radiation shield(s).

In particular, for high-resolution NMR methods, wherein even small vibrations which are introduced e.g. via the cold head of a cryocooler into the system have a negative effect on the spectrum quality, the cryocooler is advantageously a pulse tube cooler, since pulse tube coolers can be operated with extremely low vibration. In principle, other cryocoolers such as e.g. Gifford-McMahon coolers can also be used.

In a particularly advantageous manner, helium can be liquefied at the coldest stage at a temperature of 4.2 K or less to provide a plurality of different applications in region of very low temperatures. The helium which is evaporated within the cryostat is liquefied at the cold stage which is freely suspended in the neck tube, and drips back into the helium container. This reduces helium loss and the number of refilling processes or permits no-loss operation if the cooling power of the cooler is large enough. The transmission of vibrations from the cold stage to the helium container is also completely eliminated, since the coldest cold stage of the cold head is not connected to the cryostat configuration via a solid bridge.

In a preferred embodiment of the invention, the tubes of the cold head above the first cold stage and possibly also in the region of further cold stages are surrounded by thermal insulation to eliminate or at least reduce undesired heat input from the neck tube into the tubes of the cold head. The tubes above the first cold stage of the cold head have temperatures between room temperature and the temperature of the first cold stage.

In a special embodiment, the width of the gas gap can be adjusted. The temperature of the radiation shield can thereby be adjusted as desired.

The areas of the opposing surfaces which delimit the gas gap and transfer heat can advantageously be extended by providing fins.

In an advantageous embodiment of the inventive cryostat configuration, the colder heat transfer surface is rigidly connected to the cold stage;of the cryocooler cold head and is disposed above the warmer heat transfer surface to provide a precondition for forming natural convection gas flow. The warmer heat transfer surface is thereby in contact with the neck tube of the helium container.

In a further development of this embodiment, the width of the gas gap, can be enlarged until a natural convection flow is obtained in the gas gap.

Alternatively or additionally, a flow through the gas gap may be externally activated to improve heat transfer.

In a further embodiment of the inventive cryostat configuration, the radiation shield or one of the radiation shields includes a container with liquid nitrogen, wherein the nitrogen is at least partially reliquefied after evaporation due to thermal connection between the radiation shield and the cold head of the cryocooler. In this case, the radiation shield is not directly cooled by the cooler but indirectly via the evaporating nitrogen.

In a further development of this embodiment, a preferably electric heater is provided in the nitrogen container or in contact therewith, to keep the pressure in the nitrogen container at a constant value above the surrounding pressure in case of surplus cooling capacity of the cryocooler.

In a further embodiment, a preferably electric heater is provided in the helium container or in contact therewith to keep the pressure in the helium container at a constant value above the surrounding pressure in case of surplus cooling capacity of the cryocooler. It is, however, also feasible to control the power of the cooler via its operating frequency and/or the amount of the working gas (i.e. the gas pressure) in the cooler.

The advantages of the inventive cryostat configuration are utilized in a particularly favorable manner if the cryostat configuration contains a superconducting magnet arrangement, in particular, if the superconducting magnet arrangement is part of a magnetic resonance apparatus, in particular, magnetic resonance imaging (MRI) or nuclear magnetic resonance spectroscopy (NMR).

Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for describing the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic view of an inventive cryostat configuration;

FIG. 2 a shows a schematic view of a cold head of a cryocooler of an inventive cryostat configuration, which is disposed in a neck tube;

FIG. 2 b shows a schematic view of a cold head of a cryocooler of an inventive cryostat configuration, disposed in a neck tube, with contact surfaces having fins;

FIG. 3 shows a schematic view of an inventive cryostat configuration with a nitrogen tank.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an embodiment of the inventive cryostat configuration with an outer jacket 1 and a helium container 2 disposed therein. The helium container is connected to the outer jacket 1 via suspension tubes 3. A two-stage cold head 7 of a cryocooler is installed in a neck tube 4 whose upper warm end 5 is connected to the outer jacket 1 and whose lower cold end 6 is connected to the helium container 2. The helium container 2 is surrounded by a radiation shield 8 which is connected in a heat-conducting manner to the suspension tubes 3 and also to a contact surface 9 on the neck tube 4 of the helium container 2. The cold head 7 is slightly lifted to produce a gas gap 13 between a cold surface 10 on the first cold stage 11 of the cold head 7 and the neck tube 4 contact surface 9 which is connected in a heat-conducting manner to the radiation shield 8 via a fixed thermal bridge 12. Heat is transferred from the radiation shield 8 to the cold stage 11 of the cold head 7 via the narrow gas gap 13, thereby avoiding solid connection between the cold stage 11 of the cold head 7 and the radiation shield 8.

The heat {dot over (Q)} which impinges on the radiation shield 8 must be transferred through the gas gap 13 of width L to the cold head 7 of the cryocooler. For heat conduction through a stationary fluid, the following applies: ${\overset{.}{Q} = {\frac{k_{m}}{L}A\quad\Delta\quad T}}\quad$ with k_(m) being the average thermal conductivity of the fluid, A the transmission surface, and ΔT the temperature difference between the warm surface (contact surface 9) and the cold surface 10. Since the thermal conductivity of helium gas is much smaller than that of most solids, such as e.g. copper, the temperature difference between the radiation shield 8 and the first cold stage 11 of the cold head 7 increases when lifting the cold head, and the temperature of the radiation shield 8 rises. To prevent large increase in the temperature of the radiation shield 8 for a given heat flow (and thereby also prevent an increase in the heat input into the helium container 2), the distance between the two surfaces 9, 10 is advantageously kept at a minimum. If desired, the shield temperature can be easily adjusted via the width of the gas gap 13.

FIGS. 2 a and 2 b each show a cold head 7 of a cryocooler of an inventive cryostat configuration, which is disposed in a neck tube 4. FIG. 2 a shows a smooth contact surface 9, while FIG. 2 b shows an embodiment of the present invention with which the contact surface 9 is extended by additional structures 14. Such an increase can be obtained e.g. through fins or similar structures.

In the region of the first cold stage with temperatures between room temperature and the temperature of the first cold stage 11, the cold head 7 is provided with a thermal insulation 15. For cold heads with several cooling stages, the tubes of further cooling stages may also be thermally insulated.

In a further improvement, heat is transferred in the gas gap 13 through convection in addition to conduction. Convection can be forced from the outside or occurs freely if the gas gap 13 and temperature difference ΔT are sufficiently large (free or natural convection). The precondition therefore is, however, that the colder surface 10 which is in contact with the cold head 7, is disposed above the warmer surface (contact surface 9) which is in contact with the radiation shield.

The simple structure of the neck tube 4 is another advantage of the invention. Bores for screwing the contact surfaces 9 and 10 are e.g. not required. Installation and removal of the cold head 7 is possible in a simple and rapid fashion.

The radiation shield 8 may also be indirectly cooled using liquid nitrogen as shown in FIG. 3, similar to a non-actively cooled system (i.e. without cryocooler). In this case, the first cold stage 11 of the cold head 7 of the cryocooler must be connected in a heat-conducting manner to the nitrogen container 16 via a gas gap 13 such that evaporated nitrogen can be reliquefied.

There are no additional measures required to prevent the transmission of vibrations between the second cold stage 17 of the cold head 7 and the helium container 2, since the cold head 7 is freely suspended in the helium atmosphere in this region and is not in fixed contact with the helium container 2.

The inventive cryostat configuration of FIG. 3 also has a heater 18 disposed in the helium container 2 and a heater 19 in the nitrogen container 16. The heaters 18, 19 keep the pressure in the helium container 2 and in the nitrogen container 16 at a constant value above the surrounding pressure. As an alternative to arrangement of the heaters 18, 19 in the helium container 2 or on the nitrogen container 16 (FIG. 3), the heaters 18, 19 may also be disposed outside of the containers as long as thermal contact with the respective liquids is provided.

The inventive cryostat configuration permits coupling between the cold stages of the cold head 7 of the cryocooler and the cryostat configuration, wherein no detectable vibrations of the cold stages of the cold head 7 get into the cryostat, while nevertheless ensuring sufficient heat transfer. The cryostat configuration is therefore particularly suited for cooling a magnet arrangement 20 which is part of an apparatus for magnetic resonance, in particular magnetic resonance imaging (MRI) or nuclear magnetic resonance spectroscopy (NMR).

LIST OF REFERENCE NUMERALS

-   1 outer jacket -   2 helium container -   3 suspension tubes -   4 neck tube -   5 upper warm end of the neck tube -   6 lower cold end of the neck tube -   7 cold head of a cryocooler -   8 radiation shield -   9 contact surface -   10 cold surface -   11 first cold stage of the cold head -   12 thermal bridge -   13 gas gap -   14 structures -   15 thermal insulation -   16 nitrogen container -   17 second cold stage of the cold head -   18 heater in or on the helium container -   19 heater in or on the nitrogen container -   20 magnet arrangement 

1. A cryostat configuration for keeping liquid helium, the cryostat configuration comprising: an outer jacket; a helium container disposed within said outer jacket; a least two suspension tubes connected between said helium container and said outer jacket; a neck tube having an upper, warm end connected to said outer jacket and a lower, cold end connected to said helium container; a cryocooler, said cryocooler having a multi-stage cold head extending into said neck tube, wherein said outer jacket, said helium container, said suspension tubes, and said neck tube delimit an evacuated space, wherein at least one cold stage of said cold head and at least one contact surface of said neck tube define a gas gap; at least one radiation shield surrounding said helium container; and a thermal bridging means for fixed, rigid or flexible heat-conducting connection between said at least one radiation shield and at least one suspension tube as well as between said at least one radiation shield and said at least one contact surface of said neck tube, wherein heat is transferred through said gas gap from said at least one radiation shield into said at least one cold stage.
 2. The cryostat configuration of claim 1, wherein said cryocooler is a pulse tube cooler.
 3. The cryostat configuration of claim 1, wherein helium is liquefied at a temperature of 4.2 K or less at a coldest cold stage of said cryocooler cold head.
 4. The cryostat configuration of claim 1, wherein tubes of said cold head of said cryocooler, above a first cold stage and/or in a region of further cold stages, are surrounded with thermal insulation.
 5. The cryostat configuration of claim 1, wherein a width of said gas gap can be freely adjusted.
 6. The cryostat configuration of claim 1, further comprising means for increasing areas of opposing heat transferring surfaces delimiting said gas gap.
 7. The cryostat configuration of claim 6, wherein said area increasing means comprises fins.
 8. The cryostat configuration of claim 1, wherein a colder, heat-transferring surface which is rigidly connected to a cold stage of said cryocooler cold head is disposed above a warmer, heat-transferring contact surface.
 9. The cryostat configuration of claim 8, wherein a width of said gas gap can be increased until a natural convection flow occurs therein.
 10. The cryostat configuration of claim 1, wherein a gas-flow through said gas gap is externally driven to improve heat transfer.
 11. The cryostat configuration of claim 1, wherein at least said one of said radiation shields includes a container holding liquid nitrogen, wherein said nitrogen is at least partially reliquefied after evaporation due to thermal connection between said radiation shield and said cold head of said cryocooler.
 12. The cryostat configuration of claim 11, further comprising a first heater disposed in or in contact with a nitrogen container.
 13. The cryostat configuration of claim 12, wherein said first heater is an electric heater.
 14. The cryostat configuration of claim 1, further comprising a second heater disposed in or in contact with said helium container.
 15. The cryostat configuration of claim 14, wherein said second heater is an electric heater.
 16. The cryostat configuration of claim 1, wherein the cryostat configuration is structured to hold a superconducting magnet.
 17. The cryostat configuration of claim 16, wherein the superconducting magnet is part of an apparatus for magnetic resonance for magnetic resonance imaging (MRI) or for nuclear magnetic resonance spectroscopy (NMR). 